1. Field of the Invention
The present invention relates to an air-conditioning apparatus which comprises a plurality of compressors, and which controls the number of compressors driven and the capacities of the driven compressors in accordance with the air-conditioning load.
2. Description of the Related Art
In an air-conditioning apparatus comprising a plurality of indoor units, a plurality of compressors are provided for a single outdoor unit, and the number of compressors driven and the capacities of the driven compressors are controlled in accordance with the sum of the air-conditioning loads of the indoor units. An example of such an air-conditioning apparatus is disclosed in U.S. Pat. No. 4,926,652.
In the air-conditioning apparatus disclosed in this U.S. patent, each compressor is contained in a casing filled with a lubricating oil.
In general, the lubricating oil is likely to collect in the casing containing a high-capability compressor, as a result of which a lubricating oil shortage may occur in the casing containing a low-capability compressor.
To solve this problem, in one type of conventional air-conditioning apparatus, the casings of compressors are connected by means of an oil-balancing pipe, and a so-called oil-balancing operation is regularly executed, so as to permit the compressors to have different capabilities. By the oil-balancing operation, different pressures are produced in the casings and the lubricating oil is uniformly distributed between the casings. An example of such a type of air-conditioning apparatus is disclosed in U.S. Pat. No. 4,870,831.
In an air-conditioning apparatus comprising both a variable-capability compressor driven by an inverter and a fixed-capability compressor operating on a commercial power supply, the compressors are driven in combination. Where the compressors are driven in combination, the lubricating oil cannot be uniformly distributed between the compressors by the abovementioned oil-balancing operation wherein the compressors are merely permitted to have different capabilities.